EuroGamer.net reports that Microsoft was unable to comment on the rumors that Halo 2 for the PC had slipped a week. Microsoft were speaking after several leading retailers changed their listings for the game.
European dates shifted from 18th to 25th May, and American schedules went from 8th to 23rd May. It may still be a rumor, but it's unlikely for a retailer to be wrong this close to launch.